1. Field of the Invention
The present invention relates to an inkjet printing apparatus for printing characters, pictures and the like on a recording material by discharging ink onto the recording material.
2. Description of the Related Art
As shown in FIG. 1, a conventional inkjet printing apparatus includes a carriage 2 movable in a main scanning direction A, to which an ink head 1 can be attached. The inkjet printing apparatus further includes a platen roller 6 extending in the main scanning direction A for feeding a recording material (not shown), in a sub-scanning direction B perpendicular to the main scanning direction A. As shown in FIG. 2, the ink head 1 is attachable to the carriage 2 by opening a lock lever 4 pivotally attached to a bottom 2a of carriage 2, placing positioning pins 3 projecting from the bottom 2a of carriage 2 and positioning grooves 7 in the ink head 1 in engagement with each other, and closing the lock lever 4 to engage a projection 10a of lock lever 4 with a recess 10b formed in an upper position of ink head 1.
The carriage 2 has a group of electrical connection terminals 8 arranged on a surface of bottom 2a thereof. The ink head 1 also has a group of similar electrical connection terminals to those of carriage 2 arranged on a bottom surface thereof opposed to the bottom 2a. When the ink head 1 is attached to the carriage 2, the groups of connection terminals contact each other to become conductive. A printing operation is carried out based on information to be printed, printing control information and so on applied to the group of connection terminals 8 of carriage 2 through a cable 9 for the ink head. As shown in FIGS. 3A and FIG. 3B, the ink head 1 includes nozzle groups 11 each having a plurality of nozzles 12. The nozzles 12 are arranged vertically in a row, and are equal in diameter.
FIG. 13 is a block diagram showing an electrical construction of a conventional inkjet printing apparatus. The inkjet printing apparatus has a circuit board 41 mounted in the carriage, and a circuit board 42 mounted in the ink head. The circuit boards 41 and 42 are electrically connected to each other through the cable 9. The circuit board 41 in the carriage includes a CPU (central processing unit) 43, an interface control unit 44, an interface memory 45 and a print memory 46. The circuit board 42 in the ink head includes print head drivers 47B, 47C, 47M and 47Y.
Print information taken in by the interface control unit 44 is temporarily stored in the interface memory 45 according to input timing of the information. When print information for one line is stored, the information is transferred to the print memory 46 in a short time. Then, the ink head 1 is moved in the main scanning direction A. According to positions of the moved ink head 1, information such as on a print starting position and a particular pitch position is inputted to CPU 43, which processes and converts this information into information on a printing resolution. According to timing of the information processing and conversion, the print information stored in the print memory 46 is applied to the print head drivers 47B, 47C, 47M and 47Y. The print head drivers 47B, 47C, 47M and 47Y cause inks to be discharged from the nozzles 12.
For example, Japanese Unexamined Patent Publication JP-A 7-156391 (1995) discloses a technique for defining a relationship of arrangement of a plurality of nozzles. Thereby, while equally dividing drive is carried out by shifting printing timing for each ink channel provided with a nozzle, printing resolution in a sub-scanning direction is compensated by providing a plurality of rows composed of a plurality of nozzles arranged on the same line.
Japanese Unexamined Patent Publications JP-A 3-290263 (1991) and JP-A 5-96728 (1993) disclose techniques for rendering an angle of nozzle arrangement variable with respect to a transport direction of a recording material. A desired resolution is obtained by appropriately setting an angle of nozzle arrangement, an amount of the recording material fed and a moving speed of the carriage in JP-A 3-290263, or by appropriately setting an angle of nozzle arrangement, a moving speed of the carriage and ink discharge timing in JP-A 5-96728.
Further, Japanese Unexamined Patent Publication JP-A 5-201003 (1993) discloses an ink head having two types of nozzle groups different in diameter. A desired resolution is obtained by appropriately selecting a nozzle group for low resolution, a nozzle group for high resolution, or a combination of the nozzle groups. Moreover, Japanese Unexamined Patent Publication JP-A 2-26752 (1990) discloses a technique of providing a plurality of ink heads for discharging different amounts of ink, and obtaining a desired resolution by selecting one of the ink heads with an ink head selecting means.
A uniform resolution is ensured with an ink head having a single type of nozzle groups of equal diameter as disclosed in JP-A 7-156391. Different resolutions may be available from an ink head with a variable arrangement angle of nozzle groups as disclosed in JP-A 3-290263 or JP-A 5-96728. However, since the nozzles are equal in diameter and ink discharge amount therefrom are also equal, a large amount of ink is discharged for high-resolution printing, resulting in smudgy prints. For low-resolution printing, a small amount of ink is discharged to form light color prints. Further, the angle of nozzle arrangement has a great influence on printing in the sub-scanning direction, and even a slight difference in the angle results in a large difference in line feed. Consequently, a blank or overlap occurs with each line feed to lower print quality. In JP-A 5-201003 and JP-A 2-26752, the ink heads have complicated constructions and are costly to manufacture, which therefore have not come into practical use yet.
Japanese Unexamined Patent Publication JP-A 4-176660 discloses an apparatus including a slider disposed adjacent an ink discharge opening for controlling an amount of ink discharge to vary the size of ink droplets, thereby to print in a plurality of tones. This apparatus has a drawback that the resolution of prints is subject to change with the size of ink droplets